Switching Systems and Methods for Use in Uninterruptible Power Supplies

ABSTRACT

An uninterruptible power supply for providing an output power signal to a load has a ferroresonant transformer, a resonant capacitor operatively connected to the ferroresonant transformer, and an inverter operatively connected to the ferroresonant transformer. The inverter is configured to generate the output power signal based on at least one inverter control signal such that the output power signal is a quasi square wave having a first change of phase, an upper limit, and a second change of phase. The at least one inverter control signal is pulse-width modulated between the first change of phase and the upper limit, pulse-width modulated between the upper limit and the second change of phase, and held in an ON state when the output power signal is at the upper limit.

RELATED APPLICATIONS

This application, Attorney's Ref. No. P218491, is a continuation of U.S.patent application Ser. No. 13/352,308 filed Jan. 17, 2012, currentlypending.

U.S. patent application Ser. No. 13/352,308 filed Jan. 17, 2012, claimsbenefit of U.S. Provisional Patent Application Ser. No. 61/435,317 filedJan. 23, 2011, now expired.

The contents of the related application(s) listed above are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates the generation of a standby power signaland, more specifically, to uninterruptible power supply systems andmethods that generate a standby signal using an inverter system.

BACKGROUND

Uninterruptible power supplies (UPS's) have long been used to provide atleast temporary auxiliary power to electronic devices. Typically, a UPSis configured to switch between a primary power source and a standbypower source as necessary to maintain constant power to a load.Typically, the primary power source for a UPS is a utility power supply,and the standby power source may take the form of a battery system. TheUPS will normally operate in a line mode in which the utility powersignal is passed to the load when the utility power signal is withinpredefined parameters. In the line mode, the UPS will typically alsocharge the battery system. When the utility power falls outside of thepredefined parameters, the UPS will switch to standby mode in which anAC signal is generated based on the energy stored in the battery system.

A class of UPS's employs a ferroresonant transformer. A ferroresonanttransformer is a saturating transformer that employs a tank circuitcomprised of a resonant winding and capacitor to produce a nearlyconstant average output even if the input to the transformer varies. Atypical UPS employing a ferroresonant transformer takes advantage of thevoltage regulating properties of a ferroresonant transformer in bothline and standby modes. In the context of a UPS, a ferroresonanttransformer thus provides surge suppression, isolation, short circuitprotection, and voltage regulation without the use of active components.

Conventionally, in line mode, a UPS employs an inverter circuitconfigured to form a switch mode power supply. An inverter circuitconfigured as a switch mode power supply typically comprises at leastone and typically a plurality of power switches that are operatedaccording to a pulse-width modulated (PWM) signal. The PWM method ofgenerating an AC signal from a DC source allows the amplitude of the ACsignal to be determined at any point in time by controlling the dutycycle at which the inverter power switches are operated. Controlling theduty cycle at which the inverter power switches are operated produces,through an output LC filter, a desired net average voltage. Typically,the parameters of the inverter control signal are varied according to acontrol signal generated by a feedback loop having an input formed by atleast one characteristic, such as voltage, of the AC signal.

In a switch mode power supply, one of the major causes of loss ofefficiency arises from the imperfect switching characteristics of modernpower switches during the transition between the ON and OFFconfigurations of the power switches. An object of the present inventionis to provide switch mode power supplies for use in UPS systems havingimproved efficiency.

SUMMARY

The present invention may be embodied as an uninterruptible power supplyfor providing an output power signal to a load comprising aferroresonant transformer, a resonant capacitor operatively connected tothe ferroresonant transformer, and an inverter operatively connected tothe ferroresonant transformer. The inverter is configured to generatethe output power signal based on at least one inverter control signalsuch that the output power signal is a quasi square wave having a firstchange of phase, an upper limit, and a second change of phase. The atleast one inverter control signal is pulse-width modulated between thefirst change of phase and the upper limit, pulse-width modulated betweenthe upper limit and the second change of phase, and held in an ON statewhen the output power signal is at the upper limit.

The present invention may also be embodied as a method of providing anoutput power signal to a load comprising the following steps. A resonantcapacitor is operatively connected to a ferroresonant transformer. Aninverter is operatively connected to the ferroresonant transformer. Atleast one inverter control signal is supplied to the inverter such thatthe output power signal is a quasi square wave having a first change ofphase, an upper limit, and a second change of phase. The at least oneinverter control signal is pulse-width modulated between the firstchange of phase and the upper limit. The at least one inverter controlsignal is pulse-width modulated between the upper limit and the secondchange of phase. The at least one inverter control signal is held in anON state when the output power signal is at the upper limit.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a first embodiment of anuninterruptible power supply system using a ferroresonant transformersystem constructed in accordance with, and embodying, the principles ofthe present invention;

FIG. 2 is a timing diagram representing control and power signalsemployed by the UPS system depicted in FIG. 1; and

FIG. 3 depicts a first quasi-square wave form, a second quasi-squarewave form, and a third quasi-square waveform.

DETAILED DESCRIPTION

Referring initially to FIG. 1 of the drawing, depicted therein is afirst example of an uninterruptible power supply (UPS) system 20constructed in accordance with, and embodying, the principles of thepresent invention. The present invention is of particular significancewhen applied to a UPS system adapted for use in a communications system,such as CATV or telephony system, and that use of the present inventionwill be disclosed herein in detail. However, it should be understoodthat the principles of the present invention may be applied to UPSsystems adapted for use in environments other than communicationssystems.

The example UPS system 20 supplies power to a load 22 based on a primarypower signal present on an AC power line 24 (line mode) or a secondarypower signal generated by a battery pack 26 (standby mode). While theexample secondary power signal is generated by a battery pack in theexample UPS system 20, alternative power sources such as generators,fuel cells, solar cells, and the like may be used as the secondary powersource.

The example UPS system 20 comprises an input section 30, an outputsection 32, an inverter section 34, and a ferroresonant transformer 36.The example input section 30 comprises a main switch 40 and first andsecond select switches 42 and 44. The example output section 32comprises an output (e.g., resonant) capacitor 50. The output capacitor50 forms a resonant or tank circuit with the transformer 36 as will bedescribed in further detail below.

The inverter section 34 comprises an inverter circuit 60 and acontroller 62. The inverter circuit 60 may be an H-bridge circuit or anyother circuit capable of producing an appropriate AC power signal basedon a DC power signal obtained from the battery pack 26. The invertercircuit 60 is or may be conventional and will not be described herein infurther detail.

The example controller 62 controls the inverter circuit 60. Thecontroller 62 may further control the charging of the battery pack 26when the UPS system 20 operates in line mode based on temperature,voltage, and/or current signals associated with the battery pack 26.

The example inverter circuit 60 is pulse-width modulated, and theinverter section 34 functions as a switch mode power supply when the UPSsystem operates in the standby mode. As will be described in furtherdetail below, the controller 62 generates one or more inverter controlsignals that control the inverter circuit to generate a switched outputsignal.

The example ferroresonant transformer 36 comprises a core 70, inputwindings 72, an inductor 74, inverter windings 76, and output windings78. The core 70 is or may be a conventional laminate structure. Theinductor 74 defines a primary side 80 and a secondary side 82 of thetransformer 36. In the example UPS system 20, the output capacitor 50 isconnected across first and second ends 90 and 92 of the output windings78, and the load is connected between the second end 92 of the outputwindings 78 and a tap 94 in the output windings 78.

In the example transformer 36, only the input windings 72 are on theprimary side 80 of the transformer 36. The inverter windings 76 andoutput windings 78 are on the secondary side 82 of the transformer 36.In particular, the output windings 78 are arranged between the inverterwindings 76 and the inductor 74, and the inductor 74 is arranged betweenthe output windings 78 and the input windings 72. A ferroresonanttransformer appropriate for use as the example ferroresonant transformer36 is described, for example, in copending U.S. Patent Application Ser.No. 60/305,926 and Ser. No. 12/803,787, and those applications areincorporated herein by references. The principles of the presentinvention may, however, be applied to other configurations offerroresonant transformers.

In line mode, the main switch 40 is closed and the AC power line 24 ispresent on the input windings 72. The input windings 72 areelectromagnetically coupled to the output windings 78 such that aprimary AC output signal is supplied to the load 22 when the UPS system20 operates in the line mode.

In standby mode, the main switch 40 is opened, and the battery pack 26and inverter section 34 form a secondary power source supplies a standbyAC output signal to the load 22. In particular, in standby mode theinverter section 34 generates the switched power signal across theinverter windings 76, and the inverter windings 76 areelectromagnetically coupled to the output windings 78 and to the outputcapacitor such that the standby AC output signal is present across thetap 94 and the second end 92 of the output windings 78. Further, duringstandby mode, an optional switch (not shown) may be provided in serieswith the output capacitor 50 to allow the output capacitor 50 to bedisconnected from the output windings, thereby reducing peak invertercurrents observed due to charging and discharging of the outputcapacitor 50.

The example inverter section 34 conventionally comprises at a pluralityof power switches (not shown) configured as a switch mode power supply.Typically, the power switches are MOSFETS configured as an H-bridgecircuit or any other circuit capable of producing an appropriate standbyAC power signal based on a DC power signal obtained from the batterypack 26.

The inverter control module 62 generates one or more inverter controlsignals based on a characteristic, such as voltage, of the standby ACoutput signal applied to the load 22. The inverter control signal orsignals may be pulse-width modulated (PWM) signals the characteristicsof which cause the power switches of the inverter circuit 60 to open andclose as necessary to generate the standby AC output signal withinpredetermined voltage, frequency, and waveform parameters. In theexample UPS system 20 operating in standby mode, the inverter circuit60, inverter control circuit 62, the inverter windings 76, and outputwindings 78 thus form a feedback loop that controls a desired netaverage voltage as appropriate for the load 22.

The Applicants have recognized that loads, such as the example load 22to which power is supplied by a UPS used in communications networks suchas CATV networks, are constant power loads that typically employ a dioderectifier circuit supplying a large capacitor bank. Such loads demandvery high current at the peak AC power voltage at the instant the ACvoltage amplitude exceeds the bus capacitor voltage. The Applicantsfurther recognized that a substantial portion, if not all, of the loadpower will be delivered in the period during which the AC voltageamplitude is higher than the DC bus capacitor. This results in higherpeak current to compensate for the fact that less than 100% of the timeis available to transfer energy to the load.

The inverter control module 62 of the present invention thus eliminatesthe pulse-width modulation at the peak of the standby AC output signal.The Applicant has discovered that the elimination of pulse-widthmodulation at the peak of the standby AC output signal allows the powerswitches of the inverter circuit 60 to be full ON (100% duty cycle)during the time of peak current transfer to the bus capacitors.Eliminating pulse-width modulation of the inverter control signal duringat least part of the cycle of the standby AC output signal significantlyimproves (by between approximately 10-20%) the efficiency of the UPSsystem 20 when operating in standby mode.

Referring now to FIG. 2 of the drawing, depicted therein are severalwaveforms that may be implemented by the example UPS system 20 operatingin standby mode. FIG. 2 conventionally plots each voltage (y-axis)versus time (x-axis). FIG. 2 is further divided into first through ninthtime periods T₁₋₉ separated by vertical broken lines.

Depicted at 120 is an example standby AC output signal 120 supplied tothe load 22. Depicted at 130 in FIG. 2 is an example switched powersignal 130 generated by the inverter section 34 and applied across theinverter windings 76. Depicted at 140 and 142 in FIG. 2 arerepresentations of inverter control signals that may be generated by theinverter control module 62 for controlling the inverter power switchesof the inverter circuit 60. As is conventional, the first invertercontrol signal using the principles of the present invention, theinverter control signals 140 and 142 may operate at a relatively highfrequency, e.g., approximate 20 kHz, with a duty cycle that is variedbetween 0% and 100% as described below to obtain the desired waveform.

The period of peak current transfer occurs in the time periods T₂, T₅,and T₈ in FIG. 2. During these periods, the inverter control signalgenerated by the inverter control module 62 for controlling the invertercircuit 60 is held in a state that closes the power switches (100% dutycycle) of the inverter circuit 60. FIG. 2 further shows that theswitched power signal 130 generated by the example inverter section 34is pulse-width modulated (switched between OFF and ON) during the timeperiods T₁, T₃, T₄, T₆, T₇ and T₉ outside of the periods of peak currenttransfer and is held HIGH (100% duty cycle) during the time periods T₂,T₅, and T₈. The operation of these switches of the inverter circuit 60in their least efficient mode (from ON to OFF or from OFF to ON) is thusavoided during the period of peak current transfer to the load 22. Theinverter control signals 140 and 142 represent one example method ofcontrolling an inverter circuit such as the example inverter circuit 60to generate the switched power signal 130 and standby AC output signal120 as depicted in FIG. 2.

The example standby AC output signal 120 depicted in FIG. 2 is what isreferred to as a modified or quasi square wave. A standby AC powersignal having a modified or quasi square wave, such as the examplesignal 120, is appropriate for providing power to the load 22.

To provide voltage regulation, the duration of the periods of time T₂,T₅, and T₈ in which the switches are operated at 100% duty cycle (heldON) can be varied as shown in FIG. 3. FIG. 3 illustrates second andthird example standby AC power signals 150 and 160; the example standbyAC power signal 120 is also reproduced in FIG. 3 for reference. Thesecond example standby AC power signal 150 corresponds to a load havinga low DC bus relative to the mid DC bus of the load corresponding to thefirst example standby AC output signal 120. The third example standby ACpower signal 160 corresponds to a load having a high DC bus relative tothe mid DC bus of the load corresponding to the first example standby ACoutput signal 120.

Additionally, to provide voltage regulation and maintain an acceptablemodified or quasi square wave, the inverter control signals 140 and 142are generated to alter the dV/dt, or slope, of the standby AC powersignal 120 during the time periods T₁, T₃, T₄, T₆, T₇ and T₉ outside ofthe periods of peak current transfer. Additionally, the switched powersignal 130 may be held at zero during phase change transitions to allowmore control of voltage regulation.

The second example standby AC power signal 150 thus has a lower peakvoltage during peak current transfer in the time periods T₂, T₅, and T₈and steeper slope during the time periods T₁, T₃, T₄, T₆, T₇ and T₉outside of the periods of peak current transfer. The steeper slope inthe time periods T₁, T₃, T₄, T₆, T₇ and T₉ is obtained by appropriatecontrol of the duty cycle of the switched power signal 130.

The third example standby AC power signal 160, on the other hand, has ahigher peak voltage during peak current transfer in the time periods T₂,T₅, and T₈. The slope of the third example standby AC power signal issimilar to the slope of the first example AC power signal 160 during thetime periods T₁, T₃, T₄, T₆, T₇ and T₉ outside of the periods of peakcurrent transfer. However, the third example standby AC power signal 160is held at zero for a short time during crossover periods 162 and 164when the AC power signal 160 changes phase. The zero voltage at thecrossover periods 162 and 164 is obtained by turning the switched powersignal 130 OFF (0% duty cycle) during the crossover periods 162 and 164.

More generally, the switching pattern of the inverter control signalsand the design of the transformer are optimized to provide maximumefficiency across the specified output voltage and specified load range.Relevant optimization schemes include providing enough volt-seconds tothe inverter winding to meet the voltage requirements of the load butnot so many volt-seconds that the transformer saturates.

Given the foregoing, it should be apparent that the principles of thepresent invention may be embodied in forms other than those describedabove. The scope of the present invention should thus be determined bythe claims to be appended hereto and not the foregoing detaileddescription of the invention.

What is claimed is:
 1. An uninterruptible power supply for providing anoutput power signal to a load comprising: a ferroresonant transformer; aresonant capacitor operatively connected to the ferroresonanttransformer; and an inverter operatively connected to the ferroresonanttransformer, wherein: the inverter is configured to generate the outputpower signal based on at least one inverter control signal such that theoutput power signal is a quasi square wave having a first change ofphase, an upper limit, and a second change of phase; and the at leastone inverter control signal is pulse-width modulated between the firstchange of phase and the upper limit, pulse-width modulated between theupper limit and the second change of phase, held in an ON state when theoutput power signal is at the upper limit.
 2. An uninterruptible powersupply as recited in claim 1, in which a duration of the ON state isvaried to regulate the upper limit of the output power signal.
 3. Anuninterruptible power supply as recited in claim 1, in which a dutycycle of the at least one inverter control signal is varied to control aslope of the output power signal.
 4. An uninterruptible power supply asrecited in claim 1, in which: the quasi square wave of the output powersignal further has a lower limit and a third change of phase; theinverter section generates the standby power signal based on first andsecond inverter control signals; the second inverter control signal ispulse-width modulated between the second change of phase and the lowerlimit, pulse-width modulated between the lower limit and the thirdchange of phase, held in an ON state when the output power signal is atthe lower limit.
 5. An uninterruptible power supply as recited in claim1, in which the first inverter control signal is held in an OFF stateduring the changes of phase.
 6. An uninterruptible power supply asrecited in claim 1, in which: the inverter section generates the standbypower signal based on first and second inverter control signals; thefirst inverter control signal is held in an OFF state during the changesof phase; and the second inverter control signal is held in an OFF stateduring the changes of phase.
 7. A method of providing an output powersignal to a load comprising: providing a ferroresonant transformer;operatively connecting a resonant capacitor to the ferroresonanttransformer; operatively connecting an inverter to the ferroresonanttransformer; supplying at least one inverter control signal to theinverter such that the output power signal is a quasi square wave havinga first change of phase, an upper limit, and a second change of phase;and pulse-width modulating the at least one inverter control signalbetween the first change of phase and the upper limit; pulse-widthmodulating the at least one inverter control signal between the upperlimit and the second change of phase; holding the at least one invertercontrol signal in an ON state when the output power signal is at theupper limit.
 8. A method as recited in claim 7, further comprising thestep of varying a duration of the ON state to regulate the upper limitof the output power signal.
 9. A method as recited in claim 7, furthercomprising the step of varying a duty cycle of the at least one invertercontrol signal to control a slope of the output power signal.
 10. Amethod as recited in claim 7, further comprising the steps of: supplyingat least one inverter control signal to the inverter such that the quasisquare wave of the output power signal further has a lower limit and athird change of phase, and the inverter section generates the standbypower signal based on first and second inverter control signals;pulse-width modulating the second inverter control signal between thesecond change of phase and the lower limit, pulse-width modulating thesecond inverter control signal between the lower limit and the thirdchange of phase, helding the second inverter control signal in an ONstate when the output power signal is at the lower limit.
 11. Anuninterruptible power supply as recited in claim 7, further comprisingthe step of holding the first inverter control signal in an OFF stateduring the changes of phase.
 12. An uninterruptible power supply asrecited in claim 10, in which: the inverter section generates thestandby power signal based on first and second inverter control signals;the first inverter control signal is held in an OFF state during thechanges of phase; and the second inverter control signal is held in anOFF state during the changes of phase.